OFFSET
1,1
COMMENTS
Theorem (Conjectured by R. William Gosper, proved by M. D. Hirschhorn): Any sum of four distinct odd squares is the sum of four distinct even squares.
The proof uses the following identity:
(4a+1)^2+(4b+1)^2+(4c+1)^2+(4d+1)^2 = 4[ (a+b+c+d+1)^2 + (a-b-c+d)^2 + (a-b+c-d)^2 + (a+b-c-d)^2 ].
All terms == 4 (mod 8). Are all numbers == 4 (mod 8) and > 412 members of the sequence? - Robert Israel, Jul 20 2018
REFERENCES
R. William Gosper and Stephen K. Lucas, Postings to Math Fun Mailing List, July 19 2018
Michael D. Hirschhorn, The Power of q: A Personal Journey, Springer 2017. See Chapter 31.
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
MAPLE
N:= 1000: # to get all terms <= N
V:= Vector(N):
for a from 1 to floor(sqrt(N/4)) by 2 do
for b from a+2 to floor(sqrt((N-a^2)/3)) by 2 do
for c from b+2 to floor(sqrt((N-a^2-b^2)/2)) by 2 do
for d from c + 2 by 2 do
r:= a^2+b^2+c^2+d^2;
if r > N then break fi;
V[r]:= V[r]+1
od od od od:
select(t -> V[t]>=1, [$1..N]); # Robert Israel, Jul 20 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jul 19 2018
STATUS
approved